A form of electrostatic printing is one that has come to be known as direct electrostatic printing (DEP). With this method, toner particles are deposited in image configuration directly onto an image receiving medium, such as plain paper or any type of intermediate print media. A local electric field is applied between a toner source and a printhead structure. The electric field will induce a force on the toner particles which is proportional to the charge of the individual toner particles. If the electrically induced force exceeds the adhesion force, which depends on the charge of the toner particle, between the toner particle and the toner source the toner particle will be lifted off the toner source surface and accelerated towards the printhead structure. DEP printing allows simultaneous field imaging and toner transport to produce a visible image on paper directly from computer generated signals without the need for those signals to be intermediately converted to another form of energy such as light energy as is required in electrophotographic printing.
A DEP printing device has been disclosed in U.S. Pat. No. 3,689,935, issued Sep. 5, 1972 to Pressman et al.
Pressman et al. discloses a multilayered particle flow modulator comprising a continuous layer of conductive material, a segmented layer of conductive material and a layer of insulating material interposed therebetween. An overall applied field projects toner particles through apertures arranged in the modulator whereby the particle stream density is modulated by an internal field applied within each aperture.
A new concept of direct electrostatic printing was introduced in U.S. Pat. No. 5,036,341, granted to Larson, and further developed in a co-pending application.
According to Larson, a uniform electric field is produced between a back electrode and a developer sleeve coated with charged toner particles. A printhead structure, such as a control electrode matrix, is interposed in the electric field and utilized to produce a pattern of electrostatic fields which, due to control in accordance with an image configuration, selectively open or close passages in the printhead structure, thereby permitting or restricting the transport of toner particles from the developer sleeve toward the back electrode. The modulated stream of toner particles allowed to pass through the opened passages impinges upon an image receiving medium, such as paper, interposed between the printhead structure and the back electrode.
According to the above method, a charged toner particle is held on the developer surface by adhesion forces which are essentially proportional to Q.sup.2 /d.sup.2, where d is the distance between the toner particle and the surface of the developer sleeve and Q is the particle charge. The electric force required for releasing a toner particle from the sleeve surface is chosen to be sufficiently high to overcome the adhesion forces.
However, due to variations in toner particle charge, particle size and particle layer thickness, the particles are not uniformly accelerated from the developer sleeve, resulting in a relatively long train of toner particles leaving the developer sleeve and being transported towards the printhead structure. Thus, toner particles exposed to the electric field through an opened aperture are neither simultaneously released from the developer surface nor uniformly accelerated toward the back electrode. As a result, the time period from that the first particle is released until all released particles are deposited onto the image receiving medium is relatively long.
As described above, a continuous voltage signal is utilized to modulate a stream of toner particles through a pattern of apertures, opening or closing apertures during predetermined time periods (development periods) during which toner is released from the toner particle source and transported through the opened apertures to the back electrode and the image receiving medium, eg. a paper sheet. When utilizing a continuous control signal (e.g.+300 V) during a specific time period, an amount of particles is released from the particle source and exposed to an attraction force from the back electrode electromagnetic field. Particles which have gained sufficient momentum to pass through the aperture before the end of the time period are still influenced by the control signal even after passage through the aperture and are thus exposed to a divergent electromagnetic field, which may cause scattering of the toner particles. On the other hand, particles passing through the aperture immediately after the end of the time period are, during their transport from the aperture towards the back electrode, subjected to a more convergent field which provides more focused transport trajectories and thereby smaller dots. This first state prolongs the time of transport to the back electrode and also elongates the stream of toner particles, which leads to a delayed deposition on the image receiving medium and thus a lower print uniformity at lower speeds. The toner stream is also scattered, which results in larger dots and hence a lower print resolution.
This drawback is particularly critical when using dot deflection control. Dot deflection control consists of performing several development steps during each print cycle to increase print resolution. For each development step, the symmetry of the electrostatic field is modified in a specific direction, thereby influencing the transport trajectories of toner particles toward the image receiving medium. This allows several dots to be printed through each single passage during the same print cycle, each deflection direction corresponding to a new dot location. To enhance the efficiency of dot deflection control, it is particularly essential to decrease the toner transport time and to ensure direct transition from one deflection direction to another, without delayed toner deposition. For example, a 600 dpi (dots per inch) deflection control requires a dot diameter below 60 microns and high speed, eg. 10 ppm (pages per minute); dot deflection printing requires shorter toner transport time and faster transition. It is thus essential to ensure that all toner particles released from the toner source are given enough time for all toner particles to be deposited onto the image receiving medium before a transition is made from one deflection direction to another.
Therefore, in order to achieve higher speed printing with improved print uniformity, and in order to improve dot deflection control, there is still a need to improve DEP methods to allow shorter toner transport time and reduce delayed toner deposition and also lower the consumption of toner per development time unit.